DE2047875A1 - Wastewater treatment process - Google Patents

Description

Dr. F. Zumsteln sen. - Dr. E. Assmann Dr. R. Koenlgsberger - Dlpl.-Phys. R. Holzbauer - Dr. F. Zumstein Jun.

PATENT LAWYERS

TELEPHONE: COLLECTIVE NO. 225341 TELEGRAMS: ZUMPAT CHECK ACCOUNT: MUNICH 91139

BANK ACCOUNT: BANK H. HOUSES

β MUNICH 2.

D.N. 4440

STEELING DRUG INC., New York, N.T. / UNITED STATES.

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Waste water treatment process

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The present invention relates to the heat treatment of sewage sludge to improve its drainage properties.

The heat treatment of sewage sludge to improve its drainage properties is a well known process which has been practiced economically for many years. See, for example, US Pat. Nos. 1,116,953, 2,075,224, 2,131,711, 2,277,718, 2,847,379 and 3,155,611. The usual process consists in heating the sludge to temperatures of 100 to 180 ^{° C. for 30 minutes to several hours or days.} Semi-technical treatments at slightly higher temperatures and shorter times have also been reported. Cf. John Harrison "Heat Syneresis of Sewage Sludges", Water and Sewage Works, May 1968, pages 217 to 220 and the references cited therein. However, prior processes have generally aimed to heat at lower temperatures in a batch process for the time necessary to achieve the desired drainability, thereby avoiding the more expensive equipment and problems associated with a continuous process .

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In recent times, continuous heat treatments have been used. applies to the improvement of the thermal efficiency and to the increase of the process capacity. Generally used this Systems also have longer holding times (heating times), i.e. 30 minutes or more.

Although the previously known heat treatment processes achieved satisfactory improvements in the sludge dewatering properties, they gave rise to other problems, for example significantly increased coloration and higher biological oxygen demand values (BOD, Biological Oxygen Demand) of the supernatant liquid. The color is only nominally changed by the biological treatment. changes and influences the color of the sewage treatment plant effluent. This * is important because the color and BOD value are criteria for determining water quality and the strong color and BOD value of the supernatant pose particular problems for health engineers who have dealt with the heat treatment of sludge, to increase the sludge processing capacity of a sewage treatment plant.

From values obtained by heating of sludge at various temperatures during periods which extended between 10 Hinuten and 2 hours, it appeared that the color formation is directly proportional to the temperature, ie $ e higher the temperature to which the When the sludge was heated to improve its drainage properties, the higher the color value of the resulting supernatant liquid. Heating a mixture of primary and activated sludge for about 30 minutes to 150 ^° C. resulted in an APHA color value of 1200; at 180 ^° C. a color value of about 3000; at 190 ^° C. a color value of about 4100; at 200 ⁰ C a color value of about 5000 and at 210 ⁰ C a color value of about 6200. It was found that when a low color value of the supernatant liquid was desired treatment temperatures above 180 ^p C were clearly contraindicated.

The regression analysis of a series of laboratory heat treatment attempts on primary and waste activated sludge where the hold times were varied from 10 minutes to 70 minutes, and

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at which the reactor ^{temperatures were varied from 171 to 210 0} C, gave the following results:

Variable stress coefficient standard error time + 18.4 + 12.0 NS

Temperature + 50.5 ** ± 7.1

Chemical oxygen demand of the
raw mud
(COD) +26.2 + 53.4- NS

** Significant with 99% probability NS no significant effect

From these values one should conclude that over the usual range of the process only the temperature, the color of the Would affect the filtrate in a significant way.

Similarly, the analysis gave for the filtration rates for these 14 examinations the following effects:

Variable repercussion coefficient standard error time 0.0865 + 0.0694 NS

Temperature 0.1332 ** +, 0.0417

Feed solids 1.424 ♦♦ 2. 0

Again it was found that temperature is a significant factor affecting the rate of filtration. It was found, that the holding time neither the color of the filtrate nor the filter speed in the range of the usual procedures significantly influenced. In contrast to the above-mentioned effects of the holding time and the temperature, it has now been found that a substantial improvement in the sludge dewatering properties can be achieved with less color formation than before was possible by briefly heat treating the mud a much higher temperature than is normally used.

According to the present invention, a preheated waste

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sludge is briefly heated to a temperature between 190 and about 230 ⁰ C for a period of up to 240 seconds, a time that is inversely proportional to the selected temperature, whereby the dewatering properties of the insoluble pesticides are improved, while at the same time being less color and biological Oxygen demand is developed in the supernatant liquid and the sludge is cooled before too high color values develop in the supernatant liquid. Any sewage sludge can be used, such as primary, digested, activated sewage sludge, and preferably a mixture of activated and primary sludge. It can be used industrial as well as household sewage sludge. Preferred slurries are those which have a very high specific resistance to filtration, ie

7 2 have filtration values above 500 χ 10 'sec. / G, and the for practical reasons cannot be filtered on a rotary vacuum filter. Also preferred are those whose protruding Liquid have an APHA color value less than 1000.

The present invention will now be made with reference to the accompanying drawings for example explained in more detail:

1 shows a schematic flow diagram of a sewage sludge heat treatment system, which uses the inventive method ^;

FIG. 2 shows a graph of the relationship between time and temperature for color formation and for the dewatering properties of the sludge in the process according to the invention and the previously known methods; and

Figure 3 provides a graph of the relationship of time and temperature for solubilization of biological oxygen demand (BOD) of the sludge in the method according to the invention and in previously known methods.

As shown in Fig. 1, a mixture of primary and secondary sludge A with a low pressure pump 1 by a

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Grinder 2 is pumped to give a homogeneous mixture which is then pumped to a tank 3 for storage until use will. A second low-pressure pump 4 pumps a stream of the stored Sludge to a high pressure pump 7 »which the pressure of the Sludge increases to operating pressure and pumps it through the inner tube side of a U-shaped countercurrent heat exchanger 9. The flow rate of the sludge processed by the system is controlled by a variable output pump. If desired, a non-condensable gas (KGG) is available added at this point in order to improve the heat exchange efficiency of the heat exchanger. The sludge is in the exchanger heated to a temperature a few degrees below the desired final heat treatment temperature. The heated one Sludge then enters reactor 13 upstream where the sludge is at the final treatment temperature for the selected treatment time is held, this time being determined by the mud pumping speed. The mud becomes The final treatment temperature is heated by introducing the necessary amount of steam into the reactor from the steam generator 21.

The heat-treated sludge C flows through the overflow outlet line 15 from the reactor, which is attached below the upper end of the reactor 13, whereby a chamber 16 results, in which gases collect and flow out of the riser 15 as a mixture with the heated sludge. If so desired the reactor can be bypassed, so that only a part the sludge flows through the reactor by opening valve 1? partially closes and the valve 19 partially opens. The heated one Sludge then becomes one through the shell side of the U-shaped tubular heat exchanger 9 and then through a valve 45 Separator 22 passed, which separates the gas phase from the liquid phase. The gas phase is controlled via a pressure control valve 23 drained, which reduces its pressure to atmospheric pressure. A chemical agent, e.g. lime, alum, ferric chloride or other agents can be added at the entrance of the thickening tank to improve the properties of the heat-treated To further improve the sludge.

The cooled sludge is discharged from the separator 22, the liquid of which

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Liquid level regulated by a liquid control valve 24 is passed to a thickening tank 25 in which the insoluble solids can settle. The thickened sludge is then in a drainage system 27, normally a Vacuum filtration system, drained. The supernatant liquid E from the thickening tank 25 is around the drainage system 27 bypassed, increasing its capacity. The dehydrated solids F from the heat-treated thickened Sludge D becomes a solids destruction system 29 »e.g. a combustion device. The unbound liquid portion of the thickened sludge D, i.e. the supernatant Liquid E from the thickening tank and the separated liquid G from the drainage system, is pumped to a temporary storage tank 31. A small amount of air can flow through be directed to the tank to avoid the build-up of unpleasant odors and / or the development of anaerobic conditions in it, the exhaust air being directed into the aerators of the aerobic biological treatment system 33.

All or part of the effluents E and G with high biological oxygen demand from the heat-treated Sludge is then fed to a heavy duty aerobic biological treatment system 33 that is activated by the usual Mud system 35 is separate and the effluent (or effluent and sludge solids) from the heavy duty biological Treatment 33 »which is now comparable to the wastewater customary in soluble biological oxygen demand, is then entered into the input side of a conventional activated sludge system. Every part of the effluent from the storage tank 31 »der is not subjected to a separate biological treatment, during periods when the biological oxygen demand of the wastewater supplied to the system is below the average level to the activated sludge system 35 given, whereby a more constant sludge formation speed can be sustained.

Since sewage sludge tends to corrode heat exchangers, the system is provided with a solvent wash system which can be used without stopping the process

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got to. At periodic intervals a solvent is applied to the material corroding the heat exchanger surfaces, e.g. an etchant and / or detergent stored in the solvent storage tank 37 is used to clean the heat exchangers. To do this, valves 5 »11» 17 and closed, whereby the flow of the sludge is stopped to the system and the valves 41, 47 and 19 are opened, whereby the solvent is introduced into the system. The solvent flows through the internal tubing of the heat exchanger 9 through the valve 19 via the shell side of the heat exchanger 9 and then back to storage tank 37 or through valve 43 through which it is discharged into the sewage treatment system can be. If desired, in an alternative Arrangement (which is not shown) the separator can be bypassed and the entire sludge gas mixture via the control valve 24 are passed to the dilution tank 25, from which the gas is derived with the help of a Biesel tower.

As in the J? Ig. As shown in Figure 2, the time the slurry is heated is inversely proportional to the temperature chosen. For example, if the sludge is heated to 210 ° C, it should be kept at this temperature for about 30 seconds to ensure correspondingly improved drainage properties, but not longer than about 180 seconds, to avoid excessive color development of the supernatant liquid. The higher the temperature to which the sludge is heated, the narrower the range between the minimum and maximum heating time, until, for practical reasons, it is no longer possible to stay in this range when the sludge is heated to a temperature above 230 ° C will. Below 190 ⁰ C is the time which is required to improve the drainage properties satisfactorily Vfeise so long that excessive color values in the supernatant liquid can not be avoided. A heating temperature of 200 to 225 ° C for a period of between 180 seconds and 15 seconds is preferred.

For the purposes of the present invention, drainage properties of mixed primary and activated off

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falling sludge is considered to be satisfactorily improved if its vacuum filtration rate (standard conditions) is at least 48.8 kg dry solids / m ² / hour. (10 lbs. Dry solids / ft / hr.). Similarly, the dewatering properties of digested and activated sludge are considered to be satisfactorily improved when the filtration speeds.

th at least 14.6 kg dry solids / m / h. (3 lbs. Dry

solids / ft / hr.). The color of the supernatant liquid is said to be satisfactory if it is below is about 3000 APHA color units.

3 shows the relationship between the solubilization of the biological oxygen demand and the time and temperature of the heat treatment. As shown by the hatched area, conventional heat treatments lead to a biological oxygen demand in 5 days of the supernatant liquid of greater than 6g / liter, if a satisfactory improvement in filterability is achieved, surprisingly, according to the method according to the invention, the biological oxygen demand in 5 days is the supernatant Liquid only about 2 to 6 g / liter, which is a very significant improvement, since the supernatant liquid of the heat-treated sludge can increase the biological oxygen demand of the activated sludge system by up to 20 % .

The times shown in Figures 2 and 3 are the times when the sludge in the reactor is at about that in the drawings specified temperature is maintained. This time is determined by dividing the volume of the reactor by the Flow rate of the sludge stream passed through the reactor. Does not include the time during that the untreated sludge is heated in the heat exchanger and the time it takes to convert the heat-treated sludge To cool the sludge in the heat exchanger. The smaller the volume of the reactor relative to the volume of the heat exchanger is, the more significant this time is. In general, however, occurs no significant increase in the color of the supernatant after the sludge from the reactor to the Heat exchanger is passed.

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The solids concentration of the sludge is not critical, but concentrations between 2 and 8% are preferred. The slurry velocity through the heat exchangers is preferably between about 91.5 and 213> 5 cm / sec. (3 "or 7 feet per second). The slurry velocity through the reactor is preferably between 152.5" to 305 cm / min. (5 and 10 feet per minute). The reactor temperature is preferably maintained between about 185 and 23O ⁰ C, 190-210 ⁰ C,. The residence time in the reactor is maintained from 180 to 240 seconds at 19O ⁰ C and between 30 and 180 seconds at 210 ° C. The most preferred reactor temperature is about 205 ^° C. and the sludge is preferably held at this temperature for at least 30 seconds, but less than 240 seconds.

Since their presence significantly improves the heat transfer coefficient and reduces the clogging of the heat exchangers and improves the smell of the heat-treated sludge, a small amount of air, e.g. about 0.935 x * 10 "** to 9 * 35 x 10""^ nr / liter of sludge, advantageously mixed with the unheated sludge, prior to entering the heat exchanger, if only odor improvement is desired, the air can be introduced into the reactor Volume is too small to influence the chemical oxygen demand of the sludge in a noticeable way, ie the chemical oxygen demand is reduced at most by only about 1 to 3% COp, Np or a mixture of these two gases can be used.

In performing the process, about 53 "to about 79 Btu units per liter of sludge to be treated (200 to about 300 Btu per gallon) must be added to maintain the selected reactor temperature. This additional heat can be added by steam flowing directly into the The additional heat can also be supplied by indirect heat exchange with other hot fluids heated in a separate heater.

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As described above, in a preferred embodiment, the drainage liquid from the heat-treated sludge can be kept in a tank to which air is supplied to aerate the liquid. Inoculating the aerated liquid with activated sludge, or with a special sludge-forming organism that provides an easily settable sludge, results in a heavy-duty biological treatment system that can quickly reduce the biological oxygen demand of the liquid to normal wastewater areas or below. For example, with a biological oxygen demand of 4 kg BOD per day per kg sludge (MLVSS) or less, a 90 to 95% reduction in biological oxygen demand can be achieved, ie from 3 g per liter to 0.3 g per liter or ™ underneath. Such a system can be operated according to a fill-up and draw-off principle, in which the liquid that is obtained in the presence of the lamb-forming organisms is aerated, but without overflow and allows KCJi to settle once a day. The treated supernatant liquid is decanted into the activated sludge system, preferably when the biological oxygen demand of the sewage inflow is lowest. A portion of the accumulated solids is withdrawn and mixed with other sludge solids for heat treatment.

The heavy-duty biological treatment system can also be used on continuous flow basis are operated, wherein the mean) residence time is sufficient to the overflowing liquid on a to hold acceptably low value. Air is with a throughput ^ u fed into the system to achieve this reduction. The overflowing material then becomes the inflow part of the activated one Sludge system fed.

example 1

The following example is for a sludge heat treatment ' system that uses the inventive method.

A mixture of primary sludge and thickened activated sludge containing 22 g / liter of insoluble solids is used with a throughput of 11.35 liters / minute (3 gallons per minute)

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through a heat exchanger and reactor as shown in Figure 1 with capacities of 9.53 liters (2.5 gallons) (tube side) and 6.8 liters (1.8 gallons) at a pressure of 22, respectively. 1 atü (315 psi) (gauge) pumped. Sufficient steam was introduced into the reactor to keep it at a temperature of 210 ⁰ O and to provide the necessary 80 ° At for the heat exchanger. The speed of the Schi amines in the tubes of the heat exchanger was 1.07 m / sec. (3.5 ft / sec.) And 4.88 m / min. (16 ft / min.) In the reactor, resulting in a reactor residence time of 30 seconds. The cooled slurry had a vacuum filtration rate of 73-3 kg dry solids per meter per hour (15 lbs. Dry solids / ft / hr.). Only a small proportion of the insoluble solids dissolved in the process. The supernatant liquid had an APHA color value of 1200 and a biological oxygen demand of 2.2 g / liter. A usually heated sludge had a comparable rate of filtration. However, the values for the biological oxygen demand and the APHA color values of the supernatant liquids are 5 to 6 g / liter and 3000 to 4000, respectively.

Example 2

In this example, comparable conditions were retained. A mixture of primary and secondary sludge containing 40 g / liter of insoluble solids was fed through a heat exchanger with a capacity of 189 liters (50 gallons) (tube side) at a rate of 227 liters / minute (60 gal./min.) and 113.5 liters (30 gallons) (shell side) at a pressure of 31.6 kg / cm (450 psi) at a speed of 1.83 m / sec. (6 ft / sec.) Or 3.05 m / sec. (10 ft / sec.) And then pumped through a reactor, which was charged with steam, with a throughput so that the reactor ^{temperature was kept at 210 0} G, and such that a Δι in the heat exchanger of 18 ° C resulted, wherein the reactor had a capacity that a dwell? at this temperature of 30 seconds and then the sludge was passed through the shell side of the heat exchanger to obtain a sludge having a vacuum filtration rate of 73.3 kg dry solids / m / hr. (15 lbs. Dry solids / ft / hr.), The biological oxygen demand being the

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supernatant liquid 4.0 g / liter and the APHA color value of this Liquid was 2000. A sludge that has been heat-treated in the usual way has a biological need for oxygen of 7 g / liter and an APHA color value of 4000.

Example 3

In an otherwise similar approach to Example 1, with a sludge flow of 568 liters / hour (150 gallons per hour) 9.35 χ 10 "^ m ^ air / liter of sludge (1.25 standard cubic feet of air per gallon of sludge) mixed with the sludge before entering the heat exchanger. This amount of air is not enough

P off to change the biological oxygen demand of the sludge considerably through wet air oxidation, i.e. it is reduced only about 1 to 4%. The mud had a mass velocity of 41.5 cm / sec. (1.36 feet per second) at the ear inlet end and 54.6 cm / sec. (1.79 feet per second) at the outlet end of the tube, compared to 28.4 (0.93) and 30.8 (1.01) in the absence of air. The average heat transfer coefficient in the absence of air was 70 and 108.5 in Presence of air, giving an increase of 55%. Even at 400 g / hour, the heat transfer coefficient increased from 106.5 to 142.5 »which means a 34% increase. The smell of the heat treated mud was much better

fe than that of the mud, which in the same way in the absence of Air had been heated.

In some cases, especially with heavy sludges, the heat exchangers can settle out, causing blockage problems leads. The addition of air improves the mixing in the heat exchangers and prevents this settling, Surprisingly, it has been found that the increase in heat losses negligible in the heat exchangers due to the addition of air is small.

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Claims (10)

Claims s a ss rs ss s = S =: rss ss s ss s = sss = s es 3 s as = 1. / A method for reducing the color formation and solubilizing the "biological oxygen demand in the heat treatment of sewage sludge to improve its drainage properties by heating this sludge, characterized in that the sludge for up to 240 seconds to a temperature between about 190 to about 230 ⁰ C heated quickly, the heating time being inversely proportional to the selected temperature and just sufficient, the filtering capacity of the sludge to at least 48.8 kg / m ² / hour (10 lbs / ft ² / hour) in the pail of primary sludge and to 14 To increase 6 kg / m ² / hour (3 lbs / ft ² / hour) in the case of activated digested sludge and cool the sludge before excessive color values develop in the supernatant. 2. The method according to claim 1, characterized in that that the sludge is heated to a temperature between 200 and 225 ° C for a period of time between about 180 seconds and 15 seconds will. 3. The method according to claim 2, characterized in that the sludge is ^{heated to about 210 0} C for about 30 seconds. 4. The method according to any one of the preceding claims, characterized in that the process is carried out continuously, and that a stream of the sludge under pressure from a temperature below 100 ⁰ C to a temperature above 150 ⁰ C by indirect countercurrent heat exchange with a continuous stream of the heated Sludge is preheated, the sludge is then immediately ^{heated to a temperature between about 190 0} C and about 230 ⁰ C, and the heated sludge is immediately then rapidly ^{cooled to a temperature below 100 0} C by indirect countercurrent heat exchange with a stream of unheated Sludge that is preheated. 5. The method according to claim 4, characterized in that 10 - / 178 4 20A7875 that the heat-treated sludge is stored and periodically the proportion of the supernatant liquid of the stored sludge is directed to the input of a secondary biological wastewater treatment system during a period of the day on which the burden is not great. 6. The method according to claim 5 »characterized in that that the secondary biological wastewater treatment system is an activated sludge system. 7. The method according to claim 6, characterized in that a stream of air during the storage period through the heat-treated Sludge and then through the aeration system of the activating ™ th sludge system. 8. The method according to any one of claims 4 to 7 »characterized in that the sludge in an extended heat treatment zone is heat treated, through which the sludge is passed vertically upwards to an exit which is below a gas-filled area, which serves as a liquid level regulator and as a pressure surge control. 9. The method according to any one of claims 4 to 8, characterized characterized in that the heat-to-be-treated sludge is in shape a mixture with a non-condensable gas, e.g. air, P is preheated in an amount of 0.935 x 10 "" * to 9.35 x 10 "" * τΡ of the gas per liter of sludge, which significantly increases the efficiency of the heat exchange from the heat-treated sludge to the unheated sludge. 10. The method according to any one of the preceding claims, characterized in that the sewage sludge is a mixture is made up of primary and activated sludge. / 17 8 4